Materials testing grip

ABSTRACT

A materials testing grip for tensile testing of small fibers includes a pair of inwardlyconverging surfaces into which a fiber may be easily pressed, and which in fiber tension is characterized by increased gripping force.

this is a continuation of application Ser. No. 804,284, filed Dec. 9,1991, now abandoned.

FIELD OF THE INVENTION

This invention relates to materials testing grips, and more particularlyto such grips especially useful in connection with tensile testing ofsingle fiber specimens of very small diameter.

BACKGROUND OF THE INVENTION

Prior art grips used with very small diameter (e.g., 10 to 50 microns)filaments have included grips with flat rubber gripping pad surfaces,such as the pneumatic grips sold by the assignee hereof, InstronCorporation, Canton, Mass., under the designation Model 2712, and themechanically actuated grips sold by said assignee under the designationModel 2711. Such prior art grips have also included grips in which afiber is wrapped around a conical "horn" and then pressed thereagainst,as in devices sold by said assignee under the designation Model 2714.

SUMMARY OF THE INVENTION

I have discovered that a grip may be desirably provided by inclusion ofsurfaces at an angle to each other to permit easy transverseintroduction of a specimen therebetween owing to force applied normal(transverse of) to the specimen, with increased frictional resistance tolongitudinal movement of the specimen as the specimen moves transverselyfurther into the zone between the surfaces owing to tension thereonlongitudinally.

In preferred embodiments, the surfaces are provided by abuttingtorus-shaped elements, or turns of an element circular in cross-section,held in axial compression.

PREFERRED EMBODIMENTS

Preferred embodiments are shown in the drawings, and described instructure and operation.

DRAWINGS

FIG. 1 is a sectional view, taken at 1--1 of FIG. 2, of the presentlymost preferred embodiment of the invention.

FIG. 2 is a sectional view, taken at 2--2 of FIG. 1, of said embodiment.

STRUCTURE

There is shown in FIGS. 1 and 2, mounted on mounting rod 10 (which is inturn connected, in a way well known, to an upper load cell, not shown),towhich it is secured by set screw 12, a fiber grip indicated generallyat 14.

Grip 14 includes an upper annular portion 16 and an integral lowerportion 18. Secured to lower portion 18 by screw 20 is jaw support 22,which includes jaw support surface 24 (which is a portion of a torus),and threaded extension 26.

Seated in jaw support surface 24 is elastomeric jaw 28. Elastomeric Jaw30 is seated in corresponding jaw support surface 32 of second jawsupport 34, mounted on stud 36 of jaw support 22, O-ring Jaws 28 and 30being toral and of the same dimensions and in circular contacttherearound.

Threadedly engaged with portion 26 and seated against spindle shoulder38 is jaws clamp 40.

Grip 14, and its jaws 28, 30, are positioned so that the gripped fiberis coaxial with mounting rod 10 (as well as a corresponding lowermounting rod, not shown), as shown in FIGS. 1 and 2.

The O-rings 28 and 30 are in relaxed form 0.380 inches in outsidediameter and 0.250 inches in inside diameter (the hole of the doughnut);the cross-sectional diameter of the annular solid portion is 0.065inches. When assembled, as shown in FIG. 2, the 0.065 inch diameter issqueezed to0.040 inches, the cross-section going thus to (as onlydiagrammatically shown in FIG. 1) an out-of-round one.

O-rings 28 and 30 are formed of a 70-durometer butadiene acrylonitrilepolymer (also known as nitrile and Buna-N), sold for example by NorRubberCo. Inc., Bloomfield, N.J.

OPERATION

A fiber specimen is loaded into top grip 14 by holding the fiber lengthbetween two human hands and pushing it into the V-groove between jaws 28and 30, following which the top end of the fiber (not shown) is given ahalf turn around that V-groove, with hand force pressing the half turninto the groove. The lower end of the fiber is then loaded into a bottomgrip (not shown, but identical with the top grip, although oppositelyoriented, as is the custom in the art with grips generally) in the sameway, pushing the lower end of the fiber specimen into the side of thatgrip's V in line with the axis of mounting rod 10 and the placethereundercoaxial therewith where the fiber is initially gripped in theupper grip 14, and similarly taking and pressing in a half turn offiber.

The grips are then moved away from each other, in the manner familiar inthe art of tensile testing. This movement imposes further forces on thefiber driving it inwardly of the two V's, and providing even tightergripping, in both grips.

The present invention provides a number of advantages. Loading of verysmall fibers into the grips is easy. The stationary V-groove nature ofthem makes repeatability of alignment and position highly repeatablefrom test to test, especially desirable in use with opticalextensometers. Light weight of grips of the invention (e.g., 20 grams)facilitates use oflow capacity load cells. All required for loading isto place a fiber in the V; the gradual increased frictional grip aslongitudinal pulling proceeds reduces pinching and breaks at the grip.Broken specimens can easily be removed by unwinding from the groove.Actuators are not needed and parts are few and simple, reducing expense(of both manufacture and maintenance), as does the use as Jaws ofinexpensive O-rings. Coaxiality of mounting rods and gripped fiberdesirably reduce off-center loading effects.

If desired, as may in particular be so with very elastic fibers, inorder to improve gripping ability, a fiber may be wound around the gripbetween-the-jaws V.

OTHER EMBODIMENTS

The jaws may be, instead of a pair of O-rings, a unitary elastomeric Jawunit with two integral jaw portions divided by a V-groove. Or, they maybetwo or more turns of a helically wound elastomeric rod. Or, one jaw,or both jaws, may be non-elastomeric. Jaw portions need not completelyintersect at the bottom of the zone of angled relationship to define anipwithin the invention. The V-groove need not have planar jaw portions,nor need the jaw portions be symmetrical about a plane.

Other embodiments of the invention will occur to those skilled in theart.

What is claimed is:
 1. A testing grip combination comprising:an actuatorelement that is movable along an axis, a connecting member connected tosaid actuator element for movement along said axis, a first jaw portionattached to said connecting member, a second jaw portion, and means forlimiting movement of said first jaw portion and said second jaw portionthereapart, each of said first jaw portion and said second jaw portionhaving a gripping face, each said gripping face facing the other saidgripping face, the first jaw portion gripping face being out of parallelwith the second jaw portion gripping face, and defining therewith a nipthat has a portion oriented parallel to said axis so as to receive atest fiber by movement transverse to said axis.
 2. The grip combinationof claim 1 in which at least one of said gripping faces is a surface ofan element of elastomeric material.
 3. The grip combination of claim 2in which at least one of said jaw portions is an elastomeric O-ring. 4.The grip combination of claim 3 in which each of said jaw portions is anelastomeric O-ring, said element of elastomeric material being one ofsaid elastomeric O-rings.
 5. The grip combination of claim 4 whereinsaid means for limiting includes jaw supports in which respective saidO-rings are supported.
 6. The grip combination of claim 5 in which onesaid jaw support includes a stud extending through the other said jawsupport.
 7. The grip combination of claim 6 in which said stud carriesthreads which engage a nut to urge said jaw supports together tocompress said jaw portions.
 8. The grip combination of claim 1 in whichsaid jaw portions are portions of an integral helical elastomeric rod.